The pacemaker cells of mammalian heart are long spindle-shaped cells confined within the Sino-Atrial node at the base of the right atrium. The pacemaking activity of the cells has been extensively studied, but ultrastructure of membranes involved in calcium cycling is known only in sketchy. In this ultrastructural study of SAN in rabbit heart we have confirmed the existence of three cell types. The “dark” and “light” cells contain very few myofibrils and are distinguished by the dense content of intermediate filaments in the former. “Intermediate” cells, at the boundaries of the node, have more myofibrils. Dark and light cells, independently of their position, have limited content of SR elements. A scarce free SR (fSR) is limited to the peripheral third of the cell and occupies ∼ 0.04% of the cell volume. The junctional SR (jSR) forms peripheral couplings (PCs) at the cell periphery at a frequency of 0.13 /µm of sectioned cell perimeter with an average profile length of 210 nm. Using the cross sectional area of the cells as a reference we estimate that 0.43 RyR2/µm2 of fiber cross section are available. By comparison the cells of the atrium have 0.28 PC profiles/µm of sectioned fiber perimeter and additionally contain internal corbular SR. Taking PCs and corbular SR into account, we estimate that 21.75 RyR2/µm2 of cross section are available. We conclude that the content of membranes and channels involved in calcium cycling are considerably reduced in SAN cell. The implications of these findings for SAN pacemaking function are examined using a new numerical model of SAN cell biophysics. Supported by NIH 2PO1 AR 052354 (P.D Allen PI, Core D CFA) and the Intramural Research Program of NIA.